It is known that the scrap of different origin normally used in steel mills contains between about 3 and 12% of non-ferromagnetic material that is mostly made up of stony material, sand, rubber, plastic and various metals such as copper, aluminium, bronze, brass, zinc, etc. which are highly detrimental to the quality of the steel that is meant to be produced from said scrap. These pollutants cause a significant increase in power consumption, in quicklime consumption and in the production of waste, which results in a lower quality and a higher cost of the steel thus produced.
It is presently difficult to meet the requirements of European Union rules that define the criteria according to which some types of metallic scrap are no longer considered waste because the scrap being used can be small or large in size, light or heavy, homogeneous or not homogeneous and therefore a single magnetic separator is not able to effectively operate on different types of scrap.
In particular, it is difficult to clean the larger and heavier scrap usually referred to as HMS 1 or HMS 2 (acronym of the expression Heavy Metal Scrap) which consists of material from shearing, rail or naval recovery, deep drawn sheets, pieces of billets, blooms and beams, etc. This type of scrap can reach a very large size and weight in the order of several quintals or even a ton.
Known electromagnetic drums used to clean ferromagnetic scrap are normally made with two or three longitudinal polarities, i.e. extending mainly in a plane parallel to the longitudinal drum axis, that are perpendicular with respect to the feed flow of the mixed ferromagnetic material from which the inert material must be removed. A typical example of a prior art two-pole drum is disclosed in US 2009/0159511 and illustrated in FIGS. 5 and 6, that show a first solenoid 21 wound around a first pole body provided with a relevant pole shoe 22 to form a first polarity, which generates a magnetomotive force equal to about ⅔ of the total magnetomotive force of the drum. As a consequence, the remaining ⅓ is generated by the second polarity formed by a second solenoid 23 wound on a second body with a relevant shoe 24, whereas in the case of three-pole drums (e.g. DE 2007529A1, FIGS. 2 and 3) the division is about 50% of the total for the first polarity, 30-35% for the second one and 15-20% for the third one.
Both two-pole and three-pole drums are also provided with a further inactive pole body 25, of reduced section and without any solenoid wound thereon, which is arranged beyond the active polarities (in the direction of rotation of the drum) and only has the function of cancelling the magnetic field to facilitate the release of the lighter ferromagnetic material. The operational arc of the magnetic field CM generated by the drum is usually of about 180° in the circumferential direction, with the axis of attraction a-a corresponding to the axis of greater magnetomotive force that is perpendicular to the axis of rotation and arranged at an angle α varying between 15° and 45°, depending on the design parameters, with respect to the vertical axis Y-Y in quadrant III of a Cartesian reference system XY (in the illustrated example of clockwise rotation centered in the origin).
In this case the material release zone is located in quadrant I at the cancelling pole body 25, and during the path of about 180° in the circumferential direction from the attraction zone to the release zone the attracted ferromagnetic material 26 must pass through two or three successive polarities of opposite sign. The change of polarity opposes the advancing of the ferromagnetic material 26, as readily understood also because the change of polarity is from a stronger polarity to a weaker polarity; moreover also gravity opposes the advancing that takes place upwards.
The sum of these effects that oppose the advancing results in this type of electromagnetic drums being suitable only for homogeneous and small- or medium-sized ferromagnetic scrap, such as shredded vehicles (so-called “proler”), in which the inert material to be eliminated is essentially made up of rubber, plastic and non-magnetic metals with a similar size and most of the inert material 27 is removed by free fall in the attraction zone.
The remaining part of the inert material 27, generally lighter and trapped by the ferromagnetic material 26, is released during the change of polarity when the ferromagnetic material 26 tends to roll, this being possible because in this phase the advancing of material 26 is due to a mechanical driving carried out by longitudinal ribs 28 applied on the rotating shell 29 of the drum. These ribs 28 must simultaneously raise material 26 against gravity and overcome the opposing magnetic action at the polarity change, yet they cannot be too high otherwise they would hinder the fall of the inert material and would end up dragging along too much of it thus making the cleaning action ineffective.
From the above it is readily evident that this type of electromagnetic drum is not suitable to clean medium- or large-sized ferromagnetic scrap, since it has at least two kinds of drawbacks. A first drawback stems from the fact that the scrap having such a size would easily climb over ribs 28 during the polarity change, piling up in the attraction zone until seizure of shell 29. Furthermore, even in the presence of much higher ribs 28, in the above-mentioned polarity change phase the drum would require an enormous driving torque to turn over pieces weighing even some quintals that must overcome the attraction of the stronger polarity and be drawn upwards.
Another type of known electromagnetic drum, illustrated in FIGS. 7 and 8, provides on the contrary for radial pole shoes extending perpendicularly with respect to the longitudinal drum axis and therefore parallel to the feed flow of the material to be treated. In this case, radial pole shoes 31 are arranged perpendicularly to the longitudinal drum axis and circular solenoids 32 are interposed between the radial pole shoes 31 and wound on radial pole bodies 33 that coaxially enclose the drum shaft and are integrated therewith.
Still another type of known electromagnetic drum is shown in U.S. Pat. No. 2,950,008 which discloses a drum with two solenoids wound on respective pole bodies provided with pole shoes arranged at the distal ends thereof, each solenoid having a solenoid axis perpendicular to a central longitudinal axis of the drum and each pole body extending in a plane perpendicular to the drum axis.
These pole bodies are located at intermediate positions between a central pole body and two end pole bodies that have neither solenoids wound thereon nor pole shoes arranged at the distal ends thereof, said unwound pole bodies constituting regions of great magnetic dispersion. The resulting magnetic field is quite wavy in the longitudinal direction with values at the central unwound pole which are about half the values at the adjacent wound poles.
Furthermore in the drum disclosed in this document the poles are mounted on a plate that is offset from the center of the drum at a position beyond the drum axis thus resulting in a longer ferromagnetic circuit with higher dispersion. This position of the support plate is made necessary by the fact of having only two wound poles whereby in order to obtain a higher magnetic field the two solenoids must be higher, i.e. have more turns, and thus must extend beyond the drum midplane.
These other two types of drums are normally employed for an opposite function with respect to the above-described drums, namely to clean inert materials polluted by ferromagnetic material that represents a small fraction of the material to be treated.
Although in these types of drum the ferromagnetic material does not have to pass through successive polarities of opposite sign in its circumferential path around the drum, and therefore the required torque would not be too high, nonetheless they are not suitable to clean medium- or large-sized ferromagnetic scrap due to at least two kinds of drawbacks. In the first place these types of drum would require a significant oversizing of the parts to be used for this function, since they are designed to remove small amounts of ferromagnetic material, and therefore would result expensive and bulky.
Secondly, their constructive shape is magnetically dispersive and poorly effective in performing the required function in the active zone, namely on the surface of the rotating shell. In particular, in the prior art drum illustrated in FIGS. 7, 8, given the great distance between solenoids 32 and the active zone of the rotating shell 34 the dispersion of the magnetic field with such a structure can be estimated at 50-60% (it should be noted that the axis of attraction corresponding to the axis of the greatest magnetomotive force in this case coincides with the axis of rotation r-r of shell 34).
In other words, with such prior art drums the magnetic field and the magnetic field gradient are insufficient both to attract the ferromagnetic material from a distance suitable to determine an adequate fall zone for the inert material, and to draw ferromagnetic pieces weighing hundreds of kilograms and/or having a large size.